808 SECTION8Berne & Levy Physiology •Fig. 44.23 A,Endocrinesignalingleadingtotheovulationofadominantfollicleattheendofthefollicularphaseofthemenstrualcycle.B,Endocrinesignalingduringthelutealphaseofanon-pregnantmenstrualcycleleadingtothedeathofthecorpusluteumandrecruitmentoffolliclestobeginnextcycle.LATE FOLLICULAR d8-14 EARLY FOLLICULAR d3-7 Dominant follicle COC Ruptured follicle Growing large antral follicles recruited by FSH Decreasing FSH basal LH (–)(–)(+)(+)High E2 no P4 Gonado-tropes GnRH neurons GnRH neurons Kisspeptin neurons Kisspeptin neurons Moderate E2 & inhibin no P4 LH surge @ midcycle GnRH rapid pulses Gonado-tropes GnRH slow pulses • Increasing (–) feedback causing lower FSH • Follicular atresia • Selection of dominant follicle Switch to (+) feedback-especially at level of pituitary sensitivity to rapid GnRH pulses • Meiotic maturation • Luteinization • Ovulation of cumulus-oocyte complex (COC) A. Follicular Phase LATE LUTEAL (d25-28) MID-LUTEAL (d17-24) Corpus luteum from ovulated dominant follicle Large antral recruitable follicles Dying corpus luteum (corpus albicans) If no implantation no rescue by hCG Maintains active corpus luteum FSH recruits large antral follicles new cycle starts Loss of negative feedack from ovary after corpus luteum dies Gonado-tropes GnRH neurons Kisspeptin neurons GnRH rapid pulses Gonado-tropes GnRH neurons Kisspeptin neurons GnRH slow pulses (–)(–)High P4 moderate & inhibition FSH Peak Basal LH & FSH No E2 inhibin or P4 B. Luteal Phase
CHAPTER 44 The Male and Female Reproductive Systems 809 1. Corpus luteum dies, E and P levels fall. 5. Declining FSH levels progressively cause atresia of all but 1 follicle – leading to selection of dominant follicle, which produces high levels of E. 9. The corpus luteum progressively becomes less sensitive to basal LH – dies if levels of LH-like activity (i.e., hCG) do not increase. 3. FSH recruits a cohort of large antral follicles to enter rapid growth phase. Follicles secrete low amounts of E and inhibin. 7. LH surge induces meiotic maturation, ovulation and luteinization. The corpus luteum produces high P, along with E and inhibin.
4. E and inhibin negatively feed back on FSH. FSH LH Pituitary
Progesterone OvaryEstradiol-17˜2. Pituitary responds to falling E and P by increasing FSH secretion. 6. High E has positive feedback on gonadotropes— LH (and some FSH) surges. 8. High P, E and inhibin negatively feed back on LH and FSH, returning them to basal levels.
•Fig. 44.24 Thehumanmenstrualcycle,withemphasisonthe“dialogue”betweenovaryandpituitarygonadotropes.NotethattherelativechangesinthelevelsofE2andinhibinareshownbythesameline.